Define a default master server list for the root zone

Define a default master server list for the root zone

To minimize the effort required to set up IANA root zone mirroring,
define a default master server list for the root zone and use it when
that zone is to be mirrored and no master server list was explicitly
specified.  Contents of that list are taken from RFC 7706 and are
subject to change in future releases.

Since the static get_masters_def() function in bin/named/config.c does
exactly what named_zone_configure() in bin/named/zoneconf.c needs to do,
make the former non-static and use it in the latter to prevent code
duplication.